Peripheral fabrications of a bis-gold(III) complex of [26]hexaphyrin(1.1.1. 1.1.1) and aromatic versus antiaromatic effect on two-photon absorption cross section

Article abstract of DOI:10.1021/ja0744908

Peripheral modifications of bis-gold(III) [26]hexaphyrin(1.1.1.1.1.1) have been explored via perbromination and subsequent Pd-catalyzed cross-coupling reactions. β-Phenylethynylated [26]hexaphyrin complex shows the large two-photon absorption (TPA) value

Full text of DOI:10.1021/ja0744908

Published on Web 08/22/2007
Peripheral Fabrications of a Bis-Gold(III) Complex of
[26]Hexaphyrin(1.1.1.1.1.1) and Aromatic versus Antiaromatic Effect on
Two-Photon Absorption Cross Section
Shigeki Mori,† Kil Suk Kim,‡ Zin Seok Yoon,‡ Su Bum Noh,‡ Dongho Kim,*,‡ and Atsuhiro Osuka*,†
Department of Chemistry, Graduate School of Science, Kyoto UniVersity, Sakyo-ku, Kyoto 606-8502, Japan, and
Department of Chemistry, Yonsei UniVersity, Seoul 120-749, Korea
Received June 19, 2007; E-mail: osuka@kuchem.kyoto-u.ac.jp; dongho@yonsei.ac.kr
Chart 1
In recent years, there has been a growing interest in the
development of organic molecules that have large two-photon
absorption (TPA) cross-section values because of their potential
applications in diverse fields including three-dimensional (3D)
optical memory, deeper-penetrating photodynamic therapy, 3D
microfabrication, and so on.1 Design strategies toward the enhance-
ment of TPA values have been explored mainly based on multi-
chromophoric systems such as the incorporation of donor and
acceptor groups in dipolar, quadrupolar, and octupolar molecular
systems and the use of dendritic, multiply branched, three-
dimensional, and coplanar systems.2 Large TPA values are con-
sidered to arise from effectively delocalized conjugated networks,
but understanding of structural and electronic factors that lead to
large TPA still remains insufficient, requiring further systematic
studies. In this respect, expanded porphyrins are particularly
interesting and promising molecules due to their atom-economic
large TPA values.3-5 In order to enhance the TPA value of expanded
porphyrins, peripheral substitution reactions will be useful but have
not been available so far. Herein, we report the improved synthesis
and peripheral modifications of [26]hexaphyrin bis-gold(III) com-
plex 1 with the aim to enhance TPA value through expansion of
the electronic network of expanded porphyrins. These reactions are
of significant synthetic value since they can be performed in a
designed manner similarly to those of porphyrins. These studies
also allow us to examine the influence of aromatic-to-antiaromatic
change upon TPA value as a rare case by taking advantage of rigid
skeletons of hexaphyrin bis-gold(III) complexes, in which reversible
interconversions between aromatic [26]hexaphyrin and antiaromatic
[28]hexaphyrin bis-gold(III) complexes are feasible.6
Hexaphyrin bis-gold(III) complex 1 was prepared from [26]-
hexaphyrin(1.1.1.1.1.1)7 by our reported metalation protocol.6 Here,
activation of NaAuCl4 by the addition of Ag(I) salt improved the
yield of 1 to 39%. For introduction of peripheral synthetic handles,
we examined bromination of 1 with N-bromosuccinimide (NBS)
under various conditions, which did not proceed at room temper-
ature but under refluxing conditions provided mono-, di-, tri-, and
tetrabrominated hexaphyrin bis-gold(III) complexes in low yields.
These brominated products were difficult to separate due to their
similar elution behaviors either on silica gel, alumina, or GPC
column. After several experiments, we found that peripheral
bromination proceeded slowly but steadily under harsh reaction
conditions (reflux, in neat bromine, in the presence of Fe powder).
After refluxing for 3 days and subsequent usual workup, octabro-
minated product 4 was isolated as a reduced 28π-form in a high
yield (94%). Oxidation of 4 with MnO2 proceeded smoothly for
30 min to furnish corresponding [26]hexaphyrin 3 quantitatively.
The reduced form 4 is probably stabilized by the electron-deficient
bromine substituents as judged from an observation that 3 was
partially reduced during elution through a silica gel or alumina
column. The successful bromination of 1 under the harsh conditions
demonstrated its remarkable robustness.
Then, Suzuki-Miyaura coupling of 4 with phenylboronic acid
was attempted in toluene in the presence of Pd(PPh3)4 and K2CO3
for 7 days, which provided a mixture of penta-, hexa-, hepta-, and
octaphenylated products. Repeated separation by silica gel column
chromatography using a 1:4 mixture of CH2Cl2 and hexane as an
eluent and subsequent oxidation with DDQ gave 5 in 10% yield.
Stille coupling of 4 with trimethyl(phenylethynyl)tin in the presence
of Pd(PPh3)4 in THF for 24 h proceeded almost quantitatively, and
subsequent simple separation over a short silica gel column gave
octa-â-(phenylethynyl) hexaphyrin bis-gold(III) complex 7 in 90%
yield. Complexes 5 and 7 were quantitatively reduced to antiaro-
matic [28]hexaphyrin bis-gold(III) complexes 6 and 8 with NaBH4,
respectively. All of these products were fully characterized by high-
1
resolution mass spectroscopy and H and 19F NMR spectroscopy
(Supporting Information, SI). In line with the previous study,6
antiaromatic characters of 2, 4, 6, and 8 are indicated by high-field
shifted signals due to the outer NH protons appearing at 1.81, 1.06,
1.66, and 0.43 ppm, respectively.
The solid-state structures of 4, 5, and 7 (Figure 1) determined
by single-crystal X-ray diffraction analysis revealed that the two
Au(III) atoms are coordinated in the same NNCC pockets, keeping
roughly planar but considerably bent structures with large mean
plane deviations with respect to the macrocyclic 36 atoms; 0.666
Å for 4, 0.516 Å for 5, and 0.497 Å for 7, which are larger than
that of 1 (0.356 Å).8 The large distortions may be ascribed to the
steric congestion of the peripheral substituents. The Au-C and
Au-N bond lengths are quite similar for the three complexes;
1.994(10)-2.000(9) and 2.069(8)-2.087(8) Å for 4, 1.972(11)-
1.991(13) and 2.072(11)-2.090(11) Å for 5, and 1.995(8)-2.013-
(8) and 2.101(7)-2.111(7) Å for 7. While the peripheral â-phenyl
groups of 5 are attached to the hexaphyrin macrocycle with large
dihedral angles of 53.6-89.0°, most of phenylethynyl groups in 7
† Kyoto University.
‡ Yonsei University.
9
11344
J. AM. CHEM. SOC. 2007, 129, 11344-11345
10.1021/ja0744908 CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
Despite the similar number of conjugated π-electrons and the same
peripheral substituents, 5-fold decreases in the TPA value from
aromatic to antiaromatic switch are remarkable, posing an important
theoretical challenge.
In summary, we have developed effective synthetic routes to
peripherally substituted bis-gold(III) hexaphyrins starting from the
octabrominated hexaphyrin bis-gold(III) complex 4. Expansion of
π-conjugated network by the direct attachment of phenylethynyl
groups led to the enhancement of TPA value. Reversible aromatic-
to-antiaromatic changes of hexaphyrin bis-gold(III) complexes upon
two-electron reduction and oxidation allow us to quantify the
influence of the aromaticity upon TPA value for the first time.
Figure 1. X-ray crystal structures of 5 (left) and 7 (right). The thermal
ellipsoids are scaled to the 50% probability level. In the side views, meso-
pentafluorophenyl substituents are omitted for clarity.
Acknowledgment. The work at Kyoto was supported by Grants-
in-Aid for Scientific Research from MEXT. S.M. thanks the JSPS
fellowship for Young Scientists. The work at Yonsei University
was supported by BK21 and the Star Faculty Programs from the
Ministry of Education and Human Resources Development, Korea.
Table 1. TPA Values of 1-8
A/Na
λ
(nm)b
σ
(GM)
σ (N)/σ (A)
Supporting Information Available: Synthetic procedures and
spectral data (PDF); X-ray crystallographic data (CIF). This material
is available free of charge via the Internet at http://pubs.acs.org.
1
2
3
4
5
6
7
8
A
N
A
N
A
N
A
N
1260
1200
1380
1200
1380
1200
1410
1200
6100 ((500)
1200 ((300)
8600 ((500)
1600 ((500)
7200 ((500)
1700 ((500)
12700 ((500)
2600 ((500)
-
0.20
-
0.19
-
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